You already know you need water when you feel thirsty, but there are less obvious signs that you’re on your way to serious dehydration. Here's what you need to watch for.

Faster heart rate

Heart rate goes up with intensity, and it will tend to drift with long exercise (especially in the heat). However, if you’re seeing heart rates that are 15 to 20 beats higher than you’d expect, it could be your heart is compensating for reduced blood volume (aka dehydration) by beating faster.

Dizziness

You know how your head swims a bit when you suddenly stand from a sitting position? It’s called postural hypotension, and it’s the result of blood not reaching your head quickly enough as you change positions, thanks to low blood volume. If you start feeling that at the gym when switching from seated to standing workouts, it could be a sign of dehydration.

Saggy skin

Technically called “decreased skin turgor,” when the skin on the back of your hand doesn’t snap back from being pinched, it’s a sign you need more fluids, fast.

“With normal hydration, the pinched skin should return back to normal essentially immediately,” says Pickels. With moderate to severe dehydration, it will be slow to return.

“Try it now by pinching your skin for 2-3 seconds and then letting go. That’s your baseline,” he says.

If nature calls and it smells like you’ve wandered into a bus station bathroom, that’s another good sign to drink more.

“A strong urine smell can be a sign of dehydration,” says Pickels.

Of course, it’s still good that you have to pee at this point, since once you hit severe dehydration, urine production decreases dramatically. Avoid slipping into that next stage by boosting your fluid intake as soon as possible.